1. Field of the Invention
This invention relates to a process for removing a layer of tin or tin-lead alloy from the surface of a printed circuit board. More particularly, it provides, in the manufacturing process of printed circuit boards, a method for selectively removing the layer of tin or tin-lead alloy in a short period of time from the copper substrates without damaging the substrates while preventing formation of whitish precipitates in the treating solution.
2. Description of the Background Art
Conventionally, a layer of tin or tin-lead alloy on copper substrates of a circuit board is removed by the use, for example, of a mixed solution of borofluoric acid and hydrogen peroxide or nitric acid and hydrogen peroxide. Since these solutions contain a strong oxidizing agent such as hydrogen peroxide, they can remove simultaneously in a single step both the layer of tin or tin-lead alloy on the copper substrates and the intermetallic layer of tin and copper, which has been generated at the interface between the copper surface and the layer of tin or tin-lead alloy. However, such a strong oxidative action of the solution causes a serious damage to the copper substrates, especially to the edge of the pattern on the substrates. In order to alleviate this problem, an improved process for greatly decreasing the damage to the substrates by a two-step procedure using two different kinds of reagents for removal of tin and tin-lead alloy has been proposed (U.S. Pat. No. 4,687,545). Although this process could solve the above-mentioned problem to some extent, it entailed a drawback that a large amount of whitish precipitates were inevitably formed in the treating solution during long-term operation.